A heat conductive sheet has widely been used to bond a heat sink (cooling member), a heat radiation sheet, or the like, to electronic appliances in order to efficiently cool or emit heat generated inside the electronic appliances as is well known in the art. With the reduction of the sizes of electronic appliances and the improvement in the integration density of these appliances in recent years, demands for heat conductive sheets that have a high heat transfer rate, are soft and flexible during use and apply only a small load to CPU chip, etc, have increased. At the same time, the requirement for a heat conductive sheet using a non-silicone type resin (inclusive of siloxane) that is free from the occurrence of a contact defect has become greater in view of the problem of the contact defect in electronic appliances caused by silicon-containing compounds.
The sheet conductive sheets using the non-silicone type resin are described in Japanese Unexamined Patent Publication (Kokai) No. 2001-310984 and Japanese Unexamined Patent Publication (Kokai) No. 2003-238760. JP 2001-310984, for example, describes a heat conductive molding of a heat conductive composition prepared by adding 100 to 700 parts by weight of a heat conductive filler such as aluminum oxide and 400 to 900 parts by weight of soft magnetic powder to 100 parts by weight of a base resin formed from a thermoplastic elastomer (block copolymer). JP 2003-238760 describes a non-halogen flame-retardant heat radiation sheet prepared by blending 100 to 150 parts by weight of a metal hydroxide type flame retardant, 1 to 10 parts by weight of red phosphorus and 500 to 700 parts by weight of a heat conductive filler to 100 parts by weight of a binder resin formed of an ethylene acrylate polymer and an ethylene-methyl acrylate type copolymer. According to the methods described in these Patent Documents, however, the viscosity of the heat conductive composition containing the polymer component and the filler becomes extremely high when large amounts of the heat conductive filler is packed to improve heat conductivity, and kneading and molding operations become difficult when a sheet is molded from the composition.
Attempts to highly pack the heat conductive filler are also described in Japanese Unexamined Patent Publication (Kokai) No. 2004-59851, Japanese Unexamined Patent Publication (Kokai) No. 10-316953 and Japanese Unexamined Patent Publication (Kokai) No. 2004-315663, for example. JP 2004-59851, for example, describes a flame retardant, heat conductive and electric insulating pressure sensitive adhesive composition containing an alkyl(meth)acrylate type monomer, a photo polymerization initiator, heat conductive and electric insulating filler particles and a polymer type dispersant, and an adhesive sheet formed by curing (photo polymerizing) this composition by the irradiation of ultraviolet rays or radioactive rays. JP 10-316953 describes a peelable heat conductive pressure sensitive adhesive containing 100 parts by weight of a copolymer between (meth)acrylic acid alkyl ester and an ethylene type monomer capable of copolymerizing with the former, 20 to 400 parts by weight of a plasticizer and 10 to 1,000 parts by weight of a heat conductive filler, and an adhesive sheet formed by coating or transferring the adhesive. JP 2004-315663 describes an acryl type heat conductive composition containing a binder component containing a crystalline acrylic polymer consisting of a polymer of (meth)acrylic acid ester monomers having alkyl groups of at least 18 carbon atoms and a heat conductive filler, and an acryl type heat conductive sheet obtained by processing this composition into a sheet form. A heat polymerization method or a UV polymerization method can be employed for the polymerization of precursors (monomers) of the binder component.
However, the methods described above are not free from the following problems. When the heat conductive sheet formed of the polymer is acquired by polymerizing and curing the monomers, for example, it is preferred to employ the UV polymerization method as the curing method among the heat polymerization method and the UV polymerization method described above from the aspects of energy saving and easiness of control of the sheet properties. When the UV polymerization method is used, however, the problem remains in that softness of the resulting heat conductive sheet is extremely low when the heat conductive filler is packed only in a higher density. Therefore, as described in JP 10-316953, a plasticizer must be added to the heat conductive composition. To more sufficiently secure softness, the mere addition of the plasticizer is not sufficient and it is preferred to change at least 98 wt % of the monomer component contained in the (meth)acrylic type monomer or its partial polymer contained in the heat conductive composition to an alkyl(meth)acrylate type monomer having a glass transition point of a homopolymer of below −40° C. However, the cohesive strength of the surface portion of the heat conductive sheet obtained in this case is extremely low. In consequence, remainder of paste and breakage of electronic appliances are likely to occur and the sheet is extremely difficult to peel when the heat conductive sheet is bonded to the electronic appliances and is again peeled after the heat radiation plate, etc, is fixed as often employed in this technical field. Because the re-bonding operation of the heat conductive sheet is frequently carried out, the low cohesive strength of the surface portion of the heat conductive sheet is an extremely critical problem. From the feature of the UV curing method, the molecular weight of the (meth)acrylic type polymer component of the surface portion of the sheet is likely to be lower than the molecular weight of the polymer component inside the sheet when the heat conductive sheet is observed in the direction of thickness. Further, the drop of the cohesive strength at the surface portion of the sheet is more remarkable than in the heat curing method.